Sealing a fuel injector from its environment requires manufacturing processes that ensure high surface quality and surface geometry. The high-quality valve seat is produced by grinding and subsequent honing.
All subsequent processes, for instance pressing the valve seat into the valve housing or connecting the valve seat to the valve housing by a welded seam, have a detrimental effect on the quality of the surfaces and thus on the tightness of the fuel injector. In particular the roundness values at the sealing diameter, which are important for the sealing, are adversely affected. This has a negative effect on the exhaust-gas values since fuel may make its way past the sealing seat and reach the intake manifold or the combustion chamber, thereby producing an overly rich mixture, which causes poor combustion and high emissions.
Another advantage of the known fuel injectors is that a thermal connection of the valve seat to the valve sleeve is present across a large surface, which adversely affects the heat response with respect to the fuel.